Accuracy in engine control parameters has become increasingly important in reducing vehicle emissions and improving economy. One of the parameters significantly affecting emissions and economy is the timing of combustion in the cylinders of the vehicle engine. In a gas fueled engine, this timing involves the crankshaft angle location of spark. In a diesel fueled engine, the timing involves the crankshaft angle location of fuel injection.
In both gas and diesel engines, the crankshaft timing angles are referenced to the engine piston top dead center positions. Therefore, the accuracy of any control or diagnostic system for establishing or monitoring ignition timing can be no better than the accuracy of the location of piston top dead center which is the exact geometric position at which the motion of the piston and the engine cylinder reverses direction and at which the combustion chamber volume is at a minimum. It is apparent therefore that to accurately establish or monitor engine timing requires an accurate determination of the top dead center position of the pistons.
Numerous systems have been employed for providing an indication of the crankshaft angle at which the piston reaches a top dead center position. Some intrusive techniques such as the use of a dial indicator having a probe extending into the top of a cylinder, while being accurate, require access to the combustion chamber. Mechanical non-intrusive techniques have been employed which have the advantage of not requiring access to the combustion chamber but are generally inaccurate in their indication of piston top dead center. Other systems have been suggested but are generally complex in nature or do not provide the required accuracy modern engine control and diagnostic systems require.